docs: def on_touch_move(self, touch):
          """If I'm being dragged, move so as to be always positioned the same
          relative to the touch.
  
          """
          if not self.dragging:
              touch.ungrab(self)
              return
          self.pos = (
              touch.x - self.collide_x,
              touch.y - self.collide_y
          )